1. Field of the Invention
This invention relates generally to an improvement in curing advanced composites and more particularly, but not by way of limitation, to a temperature and time controlled heat system gun for curing advanced composites.
2. Description of Prior Art
In the aerospace industry the use of advanced composites is growing because of perceived advantages of strength and weight. These advanced composites may be constructed in a number of ways such as by hand lay-up, filament winding, or tape laying. Such construction includes interleaving sheets of organic fiber such as carbon or graphite with sheets of resin or preimpregnating such sheets or organic fiber with a resin. The construction is then subjected to an appropriate heat and pressure cycle to cause the resin to flow within the organic fiber sheets and to cure and harden the resin.
In a typical application of an advanced composite to an aerospace need, a special graphite fiber cord is to be attached to a series of aluminum attach points. The graphite fiber cord may be attached to each point by merely tying it to the point with a single knot and securing it in place with a thermosettable epoxy resin. To accelerate the cure of the epoxy resin, a heat gun is used. An operator of the heat gun typically manually operates the gun by holding the gun at varying distances from the workpiece, fanning the heat gun over the workpiece, and pressing the trigger of the heat gun to an "on" position as the operator perceives the need for greater or continued heat. Obviously, such a procedure yields variable results according to the skill and the interest of the operator.
If the compound cures too quickly, the epoxy resin will not be allowed to flow throughout the carbon fibers to saturate the joint thereby resulting in a weak unsatisfactory joint. On the other hand, if the compound cures too slowly, the resin will flow away from the knot and saturate the connecting cord by capillary action which action again results in a weak unsatisfactory joint.
It has also been found in the operation of an industrial heat gun in an aerospace application that operator fatigue is a significant factor. If a large number of carbon fiber/aluminum joints are to be fabricated it has been found that during the progress of the work shift and as the operator becomes fatigued, the quality of the joints is found to significantly deteriorate. Also, the experience of the aerospace worker has been found to be a significant factor in the fabrication of such joints. While an experienced worker may be far down the learning curve in the fabrication of these joints and is prepared to fabricate satisfactory joints, an unexperienced worker can be expected to produce highly variable results over what may be an extended learning period until he has acquired sufficient skill and experience. In aerospace applications, unsatisfactory results causes further complications in that costly scrap may result and preclude schedule being maintained for a project.
The need to resolve this situation existed until the emergence of the present invention which provided a temperature and time controlled heat gun system especially adapted for aerospace applications such as the curing of advanced composites. The present invention provides for automatic operation with preset controls that enable the temperature of a heated air stream to be directed against the advanced composite workpiece, the length of time the heat is to be applied, and the length of time a following cooling air stream is to be applied to said composite to be predetermined. Thus, with an automatic operation the operator of the subject heat gun system may cure any number of joints with confidence of consistency and reliability.
A review of the patent literature has uncovered some patents that are of interest. An early heat gun is disclosed in U.S. Pat. No. 1,564,896 entitled COMBINED ELECTRICAL HEATER AND BLOWER issued to S. T. Rinker et al. on Dec. 8, 1925. The Rinker et al. patent teaches a hand held electric hair dryer having a heater and electrically driven blower positioned within a pistol grip housing and arranged to permit an operator to conveniently control and manipulate the device and to effect discharge of either hot air or cold air therefrom.
U.S. Pat. No. 3,978,314 dated Aug. 31, 1976 and issued to Nobuzo Shimizu for an ELECTRIC HAIR DRYER WITH AIR BLOWING CONDITION INDICATOR discloses a hand held electric hair dryer having a handle provided with different colored windows. When a push button control switch for the dryer is activated, signal lamps are selectively lit behind the colored windows to indicate on sight the state of the air blowing from the gun and whether it is warm or not.
U.S. Pat. No. 4,260,875 dated Apr. 7, 1981 and issued to Henry J. Walter et al. for CONTROLLED TEMPERATURE HAIR DRYER discloses a hand held electric hair dryer, the temperature of the air stream flowing from which being automatically controlled by the temperature of the hair being dried. The dryer includes a primary air inlet, a fan for blowing inlet air out of the dryer, and a heater by which the blown air is heated before it is directed against a user's hair. The dryer also includes a feedback air duct and a thermistor positioned in the duct to sense the temperature of air being reflected off the user's hair as it is dried. The thermistor cooperates with an electric control system to switch off or proportionally reduce the power to the heater. Thus, the temperature of the air being blown across the user's hair is continuously monitored as the hair is dried to preclude the hair and scalp of the user from becoming overheated.
U.S. Pat. No. 3,261,397 dated July 19, 1966 and issued to Arthur D. Hay for COMBINED COOLING AND HEATING APPARATUS is also of interest. The A. D. Hay patent discloses an enclosure for electronic equipment at unattended installations that are subjected to low ambient temperatures. The subject apparatus maintains uniform temperatures within the enclosure and provides for flushing or cooling with outside air when the temperatures within the enclosure go above the desired temperatures.
In none of the above cited patents nor in any other patent reviewed in a study of the patent literature was there a disclosure of a temperature and time controlled heat gun system having the necessary controls to control the desired heat and ambient air cycles for desired durations to provide optimum curing of an advanced composite.